An optical demultiplexer (for example, Light Detecting System PSS-100 commercially available by Shimazu Seisakusho) has been known in which the light collected by a collective lens is reflected by a mirror, the reflected light is demultiplexed by a diffraction grating, and the demultiplexed 20 lights are detected by a photodetector. The photodetector employed in this″′ optical demultiplexer is a light-receiving element array and is used as a spectrum monitor for wavelength.
In this conventional photodetector, the barycenter of a 25 light intensity is monitored by dividing the spectrum of one wavelength by means of an array of light-receiving elements (e.g., five elements). Therefore, the resolution of the photodetector is determined by an array pitch of the light-receiving elements. In this manner, the conventional 30 photodetector may merely obtain the resolution corresponding to the array pitch of the light-receiving elements, so that it is difficult for the conventional photodetector to further increase its resolution. It should be noted that the barycenter of a light intensity means herein the barycenter of spectrum distribution of a wavelength.
In order to resolve the problems described above, the inventors of this application have thought that a semiconductor position sensor for detecting the position of a light spot is used as a light-receiving element array of the photodetector in the optical demultiplexer. Because the semiconductor position sensor is a non-divided type of element different from the light-receiving element array described above, the semiconductor position sensor may detect continuously and spatially the barycenter of a light intensity.
As a conventional semiconductor position sensor, the sensor has been known in which a p-type resistor layer is provided on the top surface of a high-resistance Si substrate (i-type layer), on the bottom surface thereof a n-type layer is provided, and two electrodes are provided oppositely on the p-type resistor layer.
Because surface layers form a p-n junction in this semiconductor position sensor, a photocurrent is generated due to a photoelectric effect when an incident light impinges upon the p-type resistor layer. The photocurrent generated at the position upon which an incident light impinges is divided into two currents, respective magnitude thereof being in reverse proportion to respective resistances from the light impinging position to respective two electrodes, so that the light impinging position may be detected by monitoring each current derived from respective two electrodes.
The conventional semiconductor position sensor uses a Si substrate as described above, and then has a poor sensitivity in a long-wavelength band for an optical communication. Therefore, in a case that the conventional semiconductor position sensor is used for a light-receiving element array of an optical demultiplexer, it is difficult to detect the barycenter of a light intensity of light having a long-wavelength band.